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The differential reinforcement of successive approximations, or more commonly, shaping is a conditioning procedure used primarily in behavioral psychology. It was introduced by B.F. Skinner, whom many regard as the father of behavioral psychology. In shaping, the form of an existing response is gradually changed across successive trials towards a desired target behavior using differential reinforcement. The principles of shaping are present in everyday interactions with the environment. Also, in the case of a human employing shaping to change another organism's behavior, this procedure is used when giving instructions (such as "touch the bar for food") is impossible due to the absence of language or communication between the two.

The successive approximations reinforced are increasingly accurate approximations of a response desired by a trainer. As training progresses the trainer stops reinforcing the less accurate approximations. For example, in training a rat to press a lever, the following successive approximations might be reinforced.

simply turning toward the lever will be reinforced

only stepping toward the lever will be reinforced

only moving to within a specified distance from the lever will be reinforced

only touching the lever with any part of the body, such as the nose, will be reinforced

only touching the lever with a specified paw will be reinforced

only depressing the lever partially with the specified paw will be reinforced

only depressing the lever completely with the specified paw will be reinforced

The trainer would start by reinforcing all behaviors in the first category, then restrict reinforcement to responses in the second category, and then progressively restrict reinforcement to each successive, more accurate approximation. As training progresses, the response reinforced becomes progressively more like the desired behavior.

The culmination of the process is that the strength of the response (measured here as the frequency of lever-pressing) increases. In the beginning, there is little probability that the rat would depress the lever, the only possibility being that it would depress the lever by accident. Through training the rat can be brought to depress the lever frequently.

Successive approximation should not be confused with feedback processes as feedback generally refers to numerous types of consequences. Notably, consequences can also include punishment, while shaping instead relies on the use of positive reinforcement. Feedback also often denotes a consequence for a specific response out of a range of responses, such as the production of a desired note on a musical instrument versus the production of incorrect notes. Shaping, on the other hand, involves the reinforcement of each intermediate response that further resembles the desired response.

Shaping is used in two areas in psychology: training operant responses in lab animals, and in applied behavior analysis or behavior modification to change human or animal behavious considered to be maladaptive or dysfunctional. It also plays an important role in commercial animal training.
Shaping - assists in discrimination, the ability to tell the difference between stimuli that are and are not reinforced and generalization the application of a response learned in one situation to a different but similar situation.